You are working with pixels in Magick.NET. What are the dimensions of your input images? Are they really 13392x40622? Are some of the input images bigger then that size or will you make your source image larger?

What does split into half and save again mean?

You can use a Parallel.For but Magick.NET already has some multi-threading optimizations so you will have to test for yourself which one works better.

Kindly give me an example of using Parallel For for my array of images, and also i was unable to use Evaluate to reduce red values. Say I have to reduce 10 points of red as per photoshop Color balance. How to do it using Evaluate. Also it needs to be fast.
What can be done.